The present invention relates to a small but effective electrostatic type sensor, which detects acceleration vibration of an object and converts it into an electrical signal, and more particularly to an acceleration vibration detector to be mounted in an earpiece (ear microphone), which is inserted into a human external auditory canal to detect therefrom a bone-conducted voice sound vibration generated by the wearer's speech and convert it into an electrical signal representing a voice sound.
An ear microphone mounted together with a speaker in an earpiece is shown in FIG. 1, as disclosed in pending U.S. patent application Ser. No. 428,017, filed Sept. 29, 1982, in the name of the present inventor. This earpiece enables its wearer to talk and listen simultaneously or alternately (two-way voice communication). The acceleration vibration detector of the ear microphone of FIG. 1 is a piezoelectric type. Numeral 1 designates a cylindrical cavity in a metal casing B having an ear microphone therein. Support member 2 of a plastic material is fitted into an open end portion of cavity 1 of enlarged diameter. Piezoelectric element 3 is fixedly supported in cantilever fashion by support member 2, which as positioned against a shoulder in the wall of cavity 1 formed by the enlarged diameter. Output lead wire A1 sends out voice sound electrical signals developed by the piezoelectric element 3. The balance of the structure of FIG. 1 is explained in detail later with reference to FIG. 5. The same signs and numerals in FIGS. 1 and 5 indicate the same parts of the earpiece.
Bone-conducted voice sound vibration generated by the wearer's speech is first conducted to casing B which in turn conducts the vibration through support member 2 to piezoelectric element 3. As a result, an electric signal is obtained through output lead wire A1.
The output of the piezoelectric element 3 has a frequency characteristic as shown by line "a" in FIG. 2, which has a disproportionately high peak at its intrinsic resonance frequency fo. Therefore, an ear microphone of this type has a drawback that its sensitivity is remarkably high at this frequency, whereas the sensitivity is comparatively low at the rest of the frequency range, resulting in need for more equalization processing at a later stage and more likelihood of causing a detrimental feedback at this frequency.
In addition, as shown in FIG. 1, the required axial length of element 3 limits space to be allocated for structure needed for lessening acoustical coupling (feedback) between the ear microphone and speaker. This ear microphone has other drawbacks including generation of noises inherent to the piezoelectric element and a structure difficult for quantity production due to soldering needs of very thinly stranded wires in the connections to piezoelectric element 3.